A rod portion of a flame rod used in a combustion device such as a water heater or a heat source device for a room heater is exposed to flame of a burner to be heated at a temperature of 1,000 Celsius degrees or more. Thus, when the rod portion is made of a metal material containing aluminum, low electrically conductive alumina is deposited on a surface of the rod portion by an oxidation reaction of the aluminum due to repetitive combustion of the burner. Further, when the surface of the rod portion is covered with the alumina, a flame current flowing through the flame is hardly to be transmitted to the rod portion even in a state where the burner is combusted, resulting in detection failure.
In view of the above-described circumstances, conventionally, there has been known a flame rod formed with a ceramic cover layer made of a ceramic cover material having conductivity higher than the alumina, on a surface of an insertion portion that is inserted into flame. (For example, Patent Prior Art 1: Japanese Unexamined Patent Publication No. 2003-232515 A and Patent Prior Art 2: Japanese Unexamined Utility Model Publication No. H02-007455 U) According to the conventional flame rods described above, a flame current flowing through the flame is transmitted through the ceramic cover layer to an non-insertion portion that is disposed outside the flame.
In the flame rod having the ceramic cover layer described above, the rod portion and the ceramic cover layer are different in the thermal expansion coefficients. Thus, repetitive heating and cooling may result in cracking in the ceramic cover layer or peeling-off of the ceramic cover layer. As a result, the flame current flowing through the flame is hardly transmitted from the ceramic cover layer to the rod portion. In view of the above-described circumstances, according to the Patent Prior Art 1, the thermal expansion coefficient of the ceramic cover layer is made to approximate the thermal expansion coefficient of the rod portion made of the metal, so that the cracking in the ceramic cover layer and the peeling-off of the ceramic cover layer are reduced. Further, according to the Patent Prior Art 2, an intermediate coating layer is formed between the rod portion and the ceramic cover layer, so that the cracking in the ceramic cover layer and the peeling-off of the ceramic cover layer are reduced.
However, as described in the Patent Prior Art 1, in order to make the thermal expansion coefficient of the ceramic cover layer closer to that of the rod portion, it is necessary to adjust a thickness of the ceramic cover layer with a high precision. Therefore, it is necessary to precisely manage a coating amount of the cover material forming the ceramic cover layer, resulting in lowering productivity. As described in the Patent Prior Art 2, in a case where the intermediate coating layer is formed between the rod portion and the ceramic cover layer, manufacturing time becomes longer or number of manufacturing processes becomes larger, resulting in further lowering the productivity.
Particularly, according to the conventional flame rod of the Patent Prior Art 1, in order to secure conductivity, a thickness of the ceramic cover layer is set to be 0.1 mm or more. However, a more complicated process is needed to uniformly form the ceramic cover layer having the thickness of 0.1 mm or more, resulting in further lowering the productivity. Moreover, when the ceramic cover layer has such a thickness, heat is hardly to be transferred from the ceramic cover layer to the rod portion. As a result, a difference in degrees of heat expansion between the rod portion and the ceramic cover layer becomes large, whereby there is a problem in that the cracking in the ceramic cover layer and the peeling-off of the ceramic cover layer can not be prevented effectively.